When body tissue such as the ocular fundus is irradiated with a laser, the laser is scattered by particles forming the body tissue, and an intensity distribution of reflected scattered light forms a dynamic laser speckle (random spot pattern) due to moving scattering particles such as blood cells. It is known that by detecting this pattern with an image sensor at an imaging plane, quantifying the change over time of the pattern for each pixel, and displaying it as a map, the blood flow distribution of blood capillaries in the vicinity of the body surface can be imaged.
There is a conventionally known blood flow rate measurement device employing such a phenomenon, in which blood cells of body tissue such as the ocular fundus of a subject's eye are irradiated with a laser, an image formed from light reflected from the blood cells is guided onto an image sensor such as a solid-state image pickup device (CCD camera), this image is captured and stored many times successively at predetermined time intervals, from the large number of stored images a predetermined number of images are selected, a value is calculated by integrating the change over time in the output from each pixel of each image, and the speed of blood cells (blood flow rate) is calculated from this value. In this type of blood flow rate measurement device, since the change in the output from each pixel corresponds to the speed of movement of blood cells, a blood flow rate distribution in the body tissue is displayed on a monitor screen as a two-dimensional image based on the value for the change in output from each pixel thus calculated, or the reciprocal thereof. This type of device is equipped with many calculation functions for use in clinical applications, and various improvements and modifications have been carried out for clinical applications.
(Patent Publication 1) JP-A-4-242628 (JP-A denotes a Japanese unexamined patent application publication)
(Patent Publication 2) JP-A-2003-164431
(Patent Publication 3) JP-A-2003-180641
(Non-Patent Publication 1) Keisoku-to-seigyo (Measurement and Control), Vol. 39, No. 4, pp. 246-252 (2000)
However, in the conventional device, since the laser spot on the ocular fundus is only on the order of a 3 mm square, the measurement region is limited, and in order to obtain data for blood flow over a wide area it is necessary to carry out measurement many times while changing the measurement position and combine these maps to give a large blood flow map, thus making the operation very complicated. Furthermore, for an actual patient, it is difficult to stare at one point because of impaired vision; during measurement the line of sight often moves (fixation movement), and when the measurement position deviates due to fixation movement, errors in reading a blood flow value easily occur, which is a problem.
With regard to the problem with fixation movement, the present inventors have already developed and proposed a calculation function for tracking fixation movement when fixation movement occurs during measurement by analyzing and correcting for the amount of movement, and accurately superimposing to give an average blood flow map (ref. Patent Publication 2). In this proposal, a provisional blood flow map is calculated for each screen obtained from an image sensor, and deviation of said map from an initial map is determined by calculating a spatial correlation. However, since a granular structure due to laser interference remains in the provisional blood flow map, when calculating relative positions between maps, a statistical error corresponding to the granular size occurs. That is, as long as an image captured by irradiating the ocular fundus with a laser is used, there is a limit to the performance in tracking fixation movement.